On Mon, Apr 14, 2025 at 5:21 PM Jason Merrill <ja...@redhat.com> wrote:
>
> On 3/30/25 6:12 PM, Jan Hubicka wrote:
> > Hi,
> > I noticed that this patch got forgotten and I think it may be useful to
> > solve this next stage 1.
> >
> >> cp_apply_type_quals_to_decl drops 'const' if the type has mutable members.
> >> Unfortunately TREE_READONLY on the PARM_DECL isn't helpful in the case of
> >> an
> >> invisiref parameter.
> >>
> >>>> But maybe classes with mutable
> >>>> members are never POD and thus always runtime initialized?
> >>
> >> No.
> >>
> >>> C++ frontend has
> >>>
> >>> /* Nonzero means that this type contains a mutable member. */
> >>> #define CLASSTYPE_HAS_MUTABLE(NODE) (LANG_TYPE_CLASS_CHECK
> >>> (NODE)->has_mutable)
> >>> #define TYPE_HAS_MUTABLE_P(NODE) (cp_has_mutable_p (NODE))
> >>>
> >>> but it is not exported to middle-end.
> >>>
> >>> However still this is quite special situation since the object is passed
> >>> using invisible reference, so I wonder if in this situation a copy is
> >>> constructed so the callee can possibly overwrite the mutable fields?
> >>
> >> The object bound to the invisible reference is usually a copy, mutable
> >> doesn't make a difference.
> > If I understand situation right, in the following testcase:
> >
> > struct foo
> > {
> > mutable int a;
> > void bar() const;
> > ~foo()
> > {
> > if (a != 42)
> > __builtin_abort ();
> > }
> > };
> > __attribute__ ((noinline))
> > void test(const struct foo a)
> > {
> > int b = a.a;
> > a.bar();
> > if (a.a != b)
> > __builtin_printf ("optimize me away");
> > }
> >
> > We can not assume that value of a.a was not changed by bar because a is
> > mutable, but otherwise it is safe to optimize out the final check.
> > If that is so, I think we want to let middle-end know that a type has
> > mutable field and use it here, right?
>
> Ah, yes, that makes sense.
Though I believe the front-end (cp_apply_type_quals_to_decl) already
avoids setting TREE_READONLY on const variables with mutable
subobjects, is that sufficient for the middle-end?
Jason
